The present invention relates to the art of transmitting and receiving stereophonic information on a single radio frequency signal. Specifically, a signal processing circuit for an intermediate frequency signal containing both amplitude modulation and angle modulation components is described.
A system for transmitting stereophonic related signals on the portion of the radio frequency spectrum reserved for low frequency amplitude modulation broadcast has been determined to be in the public interest by the Federal Communications Commission. Several proposals have been suggested all of which include transmitting as amplitude modulation on an R.F. carrier the summation of stereophonic related signals. A difference signal formed from the stereophonic related signals angle modulates the RF carrier, which when demodulated is combined with the amplitude demodulated signal to form first and second stereophonic signals.
The demodulation of the angularly modulated difference signal has been complicated by the presence of amplitude modulation on the signal. In the case of high negative amplitude modulation peaks, the angle demodulator is presented with a zero signal level, or marginal signal level, which when detected by the angle demodulator, can under certain signal conditions produce a noise burst.
Various techniques have been proposed for elimination or reducing the consequences of a temporary loss in signal amplitude. These include a sample and hold technique as disclosed in U.S. Pat. No. 4,340,782, and a regenerative I.F. circuit for introducing a substitute signal during those periods of signal loss as disclosed in U.S. patent application Ser. No. 159,359, filed June 13, 1980, in the name of Laurel R. Lind.
The present invention is yet another technique for minimizing the noise burst without introducing undesirable distortion products to the difference signal.